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Viscera:Rectum and anal canal
The rectum is the last part of the large intestine. Location: Within the pelvis as the continuation of the sigmoid colon after the rectosigmoid junction. In front of S3 the sigmoid colon loses it mesentery and becomes the rectum. The rectum does not have the distinctive haustra. The rectum is approximately 15 cm long. Morphology: * AP concave shape with three bends (upper and lower curves to the right and middle curve to the left) with transverse rectal folds (valves of Houston), that project into the rectum, corresponding to the concavities of the curves. * The lower part of the rectum is dilated called the rectal ampulla. * The middle fold is the largest, and is at the level at which the peritoneum is reflected off the rectum to the bladder/vagina forming the rectovescial pouch/rectouterine pouch. * The rectum continues on as the anal canal at the level of the coccyx tip as it passes through the puborectalis muscle. The puborectalis merges with the external sphincter to form a palpable ledge, called the anorectal ring. * The rectum is surrounded by "mesorectum" - connective tissue but not a mesentery. It contains the superior rectal arteries and veins/lymphatics. Relations * anteriorly: ** male - bladder, ureter, prostate, seminal vesicle ** female - bladder, ureter, vagina, rectovaginal septum * posteriorly - presacral fascia, S3-S5, coccyx, median sacral nerves, superior rectal vessels, sacral sympathetic chain * inferiorly - piriformis muscle * laterally - sigmoid colon, terminal ileum Blood supply * arterial supply ** superior rectal artery - terminal branch of inferior mesenteric artery ** middle rectal artery - inferior vesical branch of internal iliac artery ** inferior rectal artery - internal pudendal artery (branch of the internal iliac artery) * venous return - via similarly named veins to both the portal venous system and systemic venous system. There is free anastomosis forming internal rectal plexus in the submucosa and external rectal plexus outside the muscular wall. The superior drain into the IMV and inferior drain into the internal pudendal veins. Nerve supply * upper rectum - inferior mesenteric plexus * middle and lower rectum - superior and inferior hypogastric plexus Lymphatic supply * Upper rectum drain along superior rectal vessels to the inferior mesenteric group and preoartic nodes * Lower rectum pass to the internal iliac nodes along middle rectal and inferior rectal arteries to the internal iliac group. This route is minimal. Variant anatomy * imperforate rectum * fistula between the rectum and pelvic viscera The anal canal is the terminal part of the gastrointestinal tract. A muscular tube surrounded by circular muscular sphincter. * measures ~4cm long * continuous with the rectum at the anorectal junction (where the puborectalis wraps) * Divided by dentate line into upper and lower with different embryological and vascular origin. Blood supply * arterial supply ** above dentate line - superior rectal artery (from inferior mesenteric artery); small contributions from middle rectal and median sacral arteries ** below dentate line - inferior rectal artery (from internal pudendal artery) * venous drainage - continuous with rectal venous plexus (i.e. rich anastomoses) ** above dentate line - superior rectal vein to inferior mesenteric vein (portal venous system) ** below dentate line - inferior and middle rectal veins to internal iliac veins The anal canal is a site of portosystemic anastomosis. Nerve supply * above dentate line and internal anal sphincter ** sympathetic - pelvic plexus ** parasympathetic and afferent sensory - pelvic splanchnic nerves * below dentate line and external anal sphincter ** inferior rectal branches of the pudendal nerve * Autonomic nerves, sympathetic fibres from pelvic plexus, and pelvic splanchnic nerves supply the internal sphincter. * Afferent pain fibres are carried by both sympathetic and parasympathetic nerves. Lymphatic drainage * above dentate line - internal iliac nodes * below dentate line - superficial inguinal nodes Variant anatomy * imperforate rectum ** 1 in 1500-5000 newborns ** failure of the bowel to open to the external world ** may vary from stenosis to blind anal canal/rectum to absent anal canal The anal sphincter is divided into an internal and external anal sphincter. It surrounds the anal canal. Internal anal sphincter * continuation of inner rectal muscle * thickened, circular muscle fibres, up to 5 mm thick * composed of visceral muscle External anal sphincter: composed of skeletal muscle and classically categorized into deep, superficial, and subcutaneous parts but these blend into each other: * Deep part: upper rectal end ** circular muscle fibres ** blends with puborectalis part of levator ani (posteriorly and laterally), completes the ring by itself anteriorly. ** the aforementioned region is called the anorectal ring, and is palpable on rectal examination * Superficial part: '''middle of the canal ** elliptical muscle fibres ** attaches from the the tip of the coccyx posteriorly to the perineal body anteriorly via the anococcygeal ligament. The multi-layer of anococcygeal ligament, levator ani and fascia forms the postanal plate '''that the rectum lies. ** only part of the sphincter with bony attachment * subcutaneous part ** circular muscle fibres ** lower ends curve inwards, lying below the end of the internal sphincter, the intersphincteric groove is formed between the two.